The present invention relates to an optical storage medium, an optical recording method and an optical recording apparatus, in or from such a storage medium, data being recorded, erased or reproduced with irradiation of a light beam (for example, a laser beam).
Phase-change optical storage media are data-rewritable storage media, such as, recent CD-RW, DVD-RW and DVD-RAM. Especially, DVD-RW and DVD-RAM are used for recording and rewriting a large amount of data, such as video data. Phase-change optical storage media require excellent overwrite characteristics as well as recording characteristics.
Phase-change optical storage media have a structure in which at least a dielectric layer, a recording layer, another dielectric layer and a reflective layer are laminated in order on a substrate having a bottom surface to be irradiated with a laser beam carrying a recording or reproducing power, or an erasing power. The recording layer is in an amorphous phase with low reflectivity just after formed by sputtering, for example. Thus, the phase-change optical storage media are initialized, for example, with laser irradiation when shipped, so that they come into a crystalline phase with high reflectivity.
A recording method for conventional rewritable phase-change optical storage media is as follows: In a phase-change optical storage medium having such a structure, recording pulses are applied (irradiated) onto a recording layer with a laser beam having a recording power, to melt and rapidly cool down the recording layer, thus forming amorphous recorded marks thereon. Reflectivity of the recorded marks lower than that of the crystalline-phase recording layer allows optical reading of the marks as recorded data. In erasing the recorded marks, a laser beam having a power (erasing power) smaller than the recording power is irradiated onto the recording layer to raise the temperature thereof to a temperature in the range from the crystallization temperature to the melting point to change the recording layer from the amorphous phase to the crystalline phase for erasing the recorded marks, thus overwriting being enabled.
Japanese Patent No. 2962052 proposes an initialization method that gives specific reflectivities to un-recorded and recorded sections for improvements in recording density and repeated recording characteristics. However, there is no disclosure on an initialization method for optical storage media used in high linier velocity recording. In addition, the inventors of the present invention have acknowledged that the requirements disclosed in Japanese Patent No. 2962052 only cannot offer sufficient overwrite characteristics (particularly characteristics at initial overwriting) in high-speed recording to recent media with high recording density.
Japanese Unexamined Patent Publication No. 2003-162821 proposes an optical storage medium with a specific difference between the maximum erasing rate obtained by a D.C. laser with an optimum erasing power and an erasing rate obtained with an erasing power smaller than the optimum erasing power for excellent jitter and overwrite characteristics in high linear-velocity recording. Japanese Unexamined Patent Publication No. 2003-228841 proposes a recording method and an optical storage medium with an optimum erasing power based on a relationship between an erasing power used in mark formation and change in reflectivity between marks. However, the inventors of the present invention have acknowledged that the requirements disclosed in Japanese Unexamined Patent Publication No. 2003-228841 only cannot offer sufficient overwrite characteristics (particularly characteristics at initial overwriting) in high linear-velocity recording, i.e., at DVD 4×speed (linear velocity: 14 m/s) or higher.
As discussed above, the conventional optical storage media and recording methods cannot offer sufficient overwrite characteristics in high linear-velocity recording due to problems of adversely high jitters in initial overwriting and also adverse jitter characteristics in overwriting over several hundred times.